Katie and dad--I forgeo you were the original posters on this thread! Sorry.....

I'd recommend you look at the Pass First Watt amps, and those built by competent DIY'ers. All good designs mount the output semi's directly on the heatsink. Use mica + grease, or kerafoil, for insulators (they insulate the MOSFETs electrically, but are good thermal conductors). Use the largest heatsinks practical (I used Conrads, from Australia). If you MUST use a steel case, cut holes large enough to still mount the semi directly to the heatsink (i.e., don't try to sandwich the steel chassis/case between the semi and heatsink).

If you still have doubts, ask the same question on the Pass forum, indicating you are building an F5. I'm certain you'll get get a flood of consistent feedback--and that it will underscore what I'm saying.

It looks to me like the difference is dominated by K.
But I've been wrong before, and survived intact.

Sofaspud..... Fear not. You're right on the mark. And for any continued "doubters", the thermal conductivity coefficients for varous materials are readily available on the internet. In a quick nutshell: diamond is about the best (but I don't think you'll find many audio chassis made of it....!); copper is very good, aluminum is good, and steel.....is steel.

1) going through more layers *must* decrease thermal conductivity
2) steel sheet metal chassis are *non-flat* further decreasing the thermal conductivity
3) steel as has been noted several times before is a *less good* thermal conductor
4) the tabs on these plastic pak Mosfets, NOT TO-3 style transistors, are small enough and as such require the best possible thermal path
5) Most of the tabs (back surface) on the plastic paks are plated copper not steel afaik, or worse case steel plated with copper then cadnium (probably cadnium)
6) connected to a steel plate (or sheet metal) the ability of the aluminum heatsink's flat back surface to shed heat is substantially reduced (the case where there are holes punched for the devices, for example) which somewhat reduces the total ability of the heatsink to shed heat, upping the temperature rise.
6a) The lower the conductivity of the thermal path, the higher the internal temperature of the device and bonding wires can reach, for example on peak current draw or in the case of Class A devices, on turn-on. Which is why heatsinks are needed in the first place...
7) Physics: hotter devices have lower SOA.

PS, for some RF type amps a copper "spreader plate" is required because even a direct connection to the aluminum isn't sufficient.

6) connected to a steel plate (or sheet metal) the ability of the aluminum heatsink's flat back surface to shed heat is substantially reduced (the case where there are holes punched for the devices, for example) which somewhat reduces the total ability of the heatsink to shed heat, upping the temperature rise.

It would be nice to see some numbers that substantiate this. Can the heatsink shed heat into the air easier than into the steel?

edit: IOW... Without looking, I think the thermal conductivity of air is worse than steel, but ΔT may begin to dominate the equation giving air the advantage.

__________________
It is error only, and not truth, that shrinks from enquiry. - Thomas Paine

The surface to air resistance is practically unchanged for the part of the heatsink covered by the steel chassis even with high air flow, so long as the junction between them is greased. Don't worry about that. One of the problems though, is it does have to be greased all over for that, and very flat, and clamped without causing distortion. In the case of sinking a power device Through the chassis and into the aluminum extrusion, you need Much lower resistance.. to the point of necessarily removing the finish (because it's a good thermal resistor) quite far around the device. If it's epoxy or powder coat, that'll suck. It's not that the steel gives a useful "spreading" effect, it's resistance is much too high for that, but the thickness of the finish will cause distortion and voids around the edges, so it has to go away and far. Getting an even squash over the double junction with no distortion would take more and better distributed pressure than you can get by a single screw through the tab and proper torque. In the end, the extra junction, even if everthing is attented to with extreme care, makes it a loser by comparison. What's to debate?